1. Field of the Invention The present invention relates generally to fuel assemblies for a nuclear reactor and, more particularly, is concerned with instrumentation tube features for reducing coolant flow-induced vibration of a flux thimble tube within the instrumentation tube.
2. Description of the Prior Art
In a typical pressurized water nuclear reactor (PWR), the reactor core includes a large number of fuel assemblies each of which is composed of top and bottom nozzles with a plurality of elongated transversely spaced guide thimbles extending between the nozzles and a plurality of transverse grids axially spaced along the guide thimbles. Also, each fuel assembly is composed of a plurality of elongated fuel elements or rods transversely spaced apart from one another and from the guide thimbles and supported by the grids between the top and bottom nozzles.
The fuel rods each contain fissile material and are grouped together in an array which is organized so as to provide a neutron flux in the core sufficient to support a high rate of nuclear fission and thus the release of a large amount of energy in the form of heat. A liquid coolant is pumped upwardly through the core in order to extract some of the heat generated in the core for the production of useful work.
Further, in each fuel assembly, a provision is made to enable actual flux measurements to be taken under reactor operating conditions. For this purpose a hollow instrumentation tube is located in the approximate center of each fuel assembly and extends between the bottom and top nozzles. The instrumentation tube is open at its bottom end through the adapter plate of the bottom nozzle for insertion of a flux thimble tube into the instrumentation tube. The thimble tube is adapted to take the flux measurements.
Coolant flow passes upward through an annulus formed between the outside diameter of the flux thimble tube and the inside diameter of the instrumentation tube, the coolant entering this annulus from the underside of the bottom nozzle adapter plate. The coolant exits through a bleed orifice formed by the top end of the instrumentation tube and the adapter plate of the top nozzle. The coolant flow induces vibrations in the thimble tube due to the existence of radial clearance and lack of mechanical connection between the instrumentation and thimble tubes. Vibration of the thimble tube frequently results in wall degradation in both the instrumentation tube and the thimble tubes. Instrumentation tube wall degradation, in turn, increases annular coolant flow rate, thereby increasing the jetting of coolant onto surrounding fuel rods. This jetting is known to cause erosion and breakdown of the fuel rod pressure boundary
Wear-through of the tubes is presently minimized by placing a coolant flow-limiting seal between the underside of the bottom nozzle adapter plate and a nozzle attached to the core support plate. Although this has been generally successful in preventing wear-through of the tubes, it does not work in all cases. Other proposed solutions are use of small spring devices located at the entrance to the instrumentation tube. However, these devices are subject to coming loose and falling off and thus creating debris in the coolant system.
Consequently, a need exists for an alternative approach to reducing coolant flow-induced vibrations and resultant wear to the instrumentation and thimble tubes.